1. Field of the Invention
The present invention relates generally to semiconductor integrated circuit handling equipment, and more specifically to a tool for handling cassettes of semiconductor wafers during processing.
2. Description of the Prior Art
It is well known that integrated circuits must be fabricated in extremely clean environments. Particles which adhere to the surface of an integrated circuit during fabrication can completely ruin that circuit. Typically, damage caused by particulate contamination on an integrated circuit cannot be repaired, and that circuit must be discarded.
Particulate contamination is of special concern as the lithography used to define active regions and interconnect on integrated circuits becomes smaller. As integrated circuit geometries shrink below one micron in size, they become vulnerable to catastrophic contamination by ever smaller particles. If badly located, a single particle only a few microns across can completely ruin one integrated circuit.
People working in fabrication clean rooms generate a large number of particles which can contaminate the integrated circuits under fabrication. It is often estimated that 25% or more of the particulate contamination in a clean room comes from the people working there. This is true even though fabrication facility personnel wear clean garments over their normal work clothes, cover their hands, face, and hair, are air dusted clean prior to entering a clean room, and completely refrain from extremely contaminating activities such as smoking or eating in the clean room. Even when all reasonable precautions are taken, people in the clean room continue to generate a significant amount of the particulate contamination seen during integrated circuit fabrication.
Clean room designers have gone to great lengths to minimize particulate contamination. Constant air flow in a clean room, combined with extremely efficient filters, helps remove particles from the air as quickly as possible. This air flow is typically vertical from the ceiling to floor in order to minimize the length of time, and thus potential travel distance, which particles spend in the clean room.
Handling of integrated circuits by people is also minimized. Semiconductor wafers are usually transported in cassettes holding up to 25 wafers. Machines in the fabrication facility are designed to remove wafers from the cassette and replace them after processing without human interference. Other facilities have eliminated humans entirely from at least part of the work area, performing as much wafer cassette transport and handling as possible using automated methods such as robots.
Because of various cost and facility design considerations, it is not always possible to eliminate handling of wafer cassettes by humans. Even in facilities which will eventually be completely automated, cost considerations and debugging of process flows may require that integrated circuits be produced with humans handling the wafer cassettes prior to the robots being installed.
Therefore, there are many situations in which human handling of wafer cassettes is unavoidable. As cassettes are carried from place to place, it is desirable for them to be held away from the body as far as possible in order to lessen contamination. However, as the diameter of semiconductor wafers used in integrated circuit processing becomes larger, the cassettes used to carry them become heavier. This tends to cause a person carrying the cassette to hold it closer to their body, increasing potential particulate contamination.
As would be expected, experiments have confirmed that separating wafer cassettes as far as possible from people handling them lessens contamination. In fabrication facilities with a constant vertical air flow, tests have shown that air flowing at velocities of approximately 100 feet per minute or more can essentially eliminate particle contamination from a person if the wafer cassette can be separated from that person by distances of at least 12 to 16 inches at all times.
It would therefore be desirable to provide a tool for use in fabrication facilities which allows wafer cassette handling while always maintaining a minimum desired separation between the human operator and the wafer cassette. It would be further desirable for such a tool to provide for simple loading and unloading in fabrication facility machines which require that the cassette be placed in various locations and orientations.